To activate the brake, set PWM=low, and the value of the other signals doesn't seem to matter.

To leave it free-running, set PWM=high and the other inputs low.

We could pulse the PWM signal, but that would be activating the brake during the off duty-cycle.

Raspberry Pi - some GPIO outputs are high on boot

A surprise to me was that on the Pi, some of the GPIO pins (not many) are high on boot. It is a (somewhat) documented behaviour. I didn't know about that, so I've managed to wire up the robot so that one of the motors runs on boot. This is undesirable as it means the robot tries to drive away while we connect the battery.